Embodiments of the present specification relate to electron emitters, and more particularly to electron emitter devices for use in X-ray tubes.
Typically, X-ray tubes are used in non-invasive imaging systems. Non-limiting examples of such non-invasive imaging systems may include X-ray systems and computed tomography (CT) systems. In these systems, the X-ray tubes are used as a source of X-ray radiation. Further, the X-ray radiation is emitted in response to control signals transmitted during an examination or an imaging sequence. Usually, an X-ray tube includes a cathode and an anode. Further, the cathode may include an emitter. The emitter is configured to emit a stream of electrons in response to an applied electrical current, and/or an electric field resulting from an applied voltage. This stream of electrons is then directed towards the anode disposed in a path of the electron beam. Typically, the anode is in the form of a metallic plate. Additionally, the anode may include a target that is impacted by the stream of electrons. The target may produce X-ray radiation as a result of impact of the stream of electrons. The X-ray radiation is emitted toward a volume of interest in a subject that needs to be imaged.
In non-invasive imaging systems, in operation, the X-ray radiation passes through the subject, such as a patient, baggage, or an article of manufacture. Further, image data is collected when at least a portion of this X-ray radiation that passes through the subject impacts a detector or a photographic plate. In the case of digital X-ray systems, a photo-detector produces signals representative of the amount or intensity of radiation impacting discrete elements of a detector surface. Further, in the case of CT systems, a detector array, including a series of detector elements, produces detected signals through various positions as a gantry is rotated about a patient. By way of example, each detector element of the detector array produces a separate electrical signal indicative of the attenuated beam received by each detector element.
Moreover, the electrical signals produced by the detector in response to the detected radiation are processed to generate an image that may be displayed for review. Further, electrical signals may be transmitted to a data processing system for analysis. The data processing system may be configured to process the electrical signals to facilitate generation of an image of the volume of interest in the subject.
Furthermore, intensity requirements for X-ray tubes used in applications, such as computed tomography, have steadily grown with the manifold possibilities of computed tomography. For example, applications of X-rays require high intensity X-rays and smaller focal spots (FS) for higher image quality. Present day's high-end X-ray tubes directly heated flat emitters are used that are structured to define an electrical path. Further, the flat emitters are configured to obtain the required high electrical resistance. However, in directly heated flat emitters, an unavoidable temperature gradient arises due to heat dissipation through the contacts. At hottest points, referred to as “hot spots” in the emitters the material evaporates in an intensified manner. The resulting decrease in the cross-section of the current-carrying path that results leads to the failure of the emitter due to additional heating, melting or fusing.
Attempts have been made to achieve an optimally homogeneous temperature distribution on a surface of the emitter and to lower a temperature at the hot spots on the surface of the emitter. It may be noted that lowering the temperature at the hot spots may result in less material being evaporated from these hot spots, and the lifetime of the emitter being prolonged accordingly. There are chances that after a prolonged use of the emitter, the emitter may develop a fracture at the hot spots.
Further, when the flat emitters are used as an electron source in an X-ray tube application, it is desirable to lower the voltage necessary for the thermionic emitter elements to generate an electron beam, so as to lower the probability of breakdown caused by operational failures and structural wear associated with an overvoltage being applied to the gate layer. Additionally, the emitter may contain one or more surface defects. By way of example, the emitter may have surface defects occurring due to machining. Further, the surface defects may result in a change in direction and/or intensity distribution of electron beams being emitted from the emission surface, thereby adversely affecting properties, such as, a size of the focal spot, or the electron beams.
Moreover, occurrence of mechanical damage due to wear and tear is typically higher at edges of the emission surface. Further, this wear and tear renders the surface at the edges of the emission surface uneven. Consequently, electron beams emitted from the edges of the emission surface may not conform to the electron beams that are being emitted from other non-damaged portions of the emission surface. In particular, the electron beams emitted from the edges of the emission surface may be divergent in nature. These divergent electron beams may prevent the electron beams from focusing onto a small, useable focal spot on the anode. Accordingly, such wear and tear prevents the electron beam from forming a small size focal spot on the target.